1. Field of the Invention
The invention relates to a novel plastisol composition based on styrene copolymers, plasticizers and inorganic fillers and also, where appropriate, other normal additions.
The term plastisols is understood to refer generally to dispersions of organic plastics in plasticizers which gel when heated to a higher temperature and harden upon cooling. The plastisols which are still customary in practice today quite predominantly contain finely powdered polyvinyl chloride (PVC) which is dispersed in a liquid plasticizer and forms a paste. Such polyvinyl chloride plastisols are used for many different purposes. They are used inter alia as sealing compounds, e.g. for seam seals in metal containers or as flange-joint adhesives in the metal industry, as corrosion-protection coverings for metals (for example as undersealing for motor vehicles), for the impregnation and coating of substrates made from textile materials (e.g. as carpet-back coatings), as cable insulation etc.
However, a series of problems occur during the production and use of PVC plastisols. The production of PVC itself is not without problems, because those who work in the production sites are exposed to a health risk from the vinyl chloride monomer. Residues of vinyl chloride monomer in the PVC could also pose a threat to health during further processing or to end-users, although the contents are generally only in the ppb range.
What is particularly serious when using PVC plastisols is that PVC is both heat- and light-sensitive and tends towards the splitting-off of hydrogen chloride. This represents a serious problem especially when the plastisol must be heated to a higher temperature, as the hydrogen chloride released under these conditions has a corrosive action and attacks metallic substrates. This is particularly true if, to shorten the gelation time, relatively high stoving temperatures are used or if, as during spot welding, locally high temperatures occur.
The greatest problem occurs when disposing of PVC-containing waste: dioxins, which are known to be highly toxic, can occur under certain circumstances in addition to hydrogen chloride. In conjunction with steel scrap, PVC residues can lead to an increase in the chloride content of the steel melt, which is likewise disadvantageous.
It is therefore the object of the invention to develop a polyvinyl chloride-free plastisol composition whose application properties correspond to those of PVC plastisols.
Polyurethane- or acrylate-based plastisols have already become known. Two-component polyurethane systems differ fundamentally from normal plastisols in use, the complicated plant needed to process them not, as a rule, being available to users. Single-component polyurethane systems are not sufficiently storage-stable. Attempts to deal with this drawback by encapsulating the isocyanates are very expensive, so that these products cannot then compete with PVC plastisols in terms of cost. Nor is the abrasion resistance sufficient for many applications, e.g. as undersealing for motor vehicles.
2. Discussion of Related Art
Acrylate plastisols, such as have become known through DE-B-24 54 235 and DE-B-25 29 732, largely satisfy the technical requirements mentioned initially, but the necessary acrylate polymers are much more expensive than polyvinyl chloride, so that the use of such acrylate plastisols has hitherto remained limited to special application fields, for example as spot-welding pastes, in which PVC plastisols fail totally. Plastisols based on styrene/acrylonitrile copolymers according to EP-A-261 499 likewise still fail to provide a satisfactory solution, because of their inadequate abrasion resistance and storage stability.
EP-A-265 371 describes plastisols comprising a dispersion of carboxyl group-containing, finely dispersed polymers, which are reacted with multifunctional basic substances, in plasticizers. Copolymers of any monomers with polymerizable acids, e.g. copolymers of vinyl chloride, vinylidene chloride, acrylates, methacrylates, maleinates, styrene, methyl styrene, vinyl esters, vinyl ethers, acrylonitrile, olefins or dienes with acids such as acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid or fumaric acid, are used as polymers. These copolymers are reacted with basic substances such as basic metal compounds of polyvalent metals, at least bifunctional amine compounds and others. In practical use, these plastisols cannot satisfy; the mechanical properties (elasticity or elongation at break) are inadequate. There is also a marked tendency towards discoloration and, when polyfunctional amines are added, large-pored blistering occurs upon gelation.